This invention relates generally to hybrid composite leaf springs with variable spring rates and, more particularly, to such springs and a method of manufacturing such springs by substituting or combining different proportions of fibers of different moduli.
Springs are used as pan of an apparatus, called a suspension, which functions to suspend a vehicle's running gear from the vehicle frame, whether separate from or integrated with the vehicle body. The spring suspension is thus, technically, a pan of the vehicle structure. However, it differs substantially from what is conventionally considered to be a part or component of the vehicle structure.
Because a vehicle must have some degree of flexure, as do buildings and bridges, the structural parts of vehicles exhibit some flexure. However, this flexure is strictly limited, as each vehicle is "tuned" to provide a predetermined stiffness. These structural parts are subject to compression, tension, torsion and bending forces and to various combinations of them during operation of the vehicle.
A vehicle's suspension includes many component parts (control arms, knuckles, etc.) which are relatively rigid, and springs, which, by definition, are not. Springs must have a high degree of flexure to cushion the vehicle and its occupants from shocks as the vehicle traverses non-smooth terrain. While structural pans of necessity exhibit little flexure and minute deflection to maintain vehicle stiffness, springs must exhibit great flexure and large deflections to provide ride smoothness.
Many types of springs are used in vehicle suspensions. Multi-leaf steel springs have often been used for rear suspensions, while steel coil springs have been used for front suspensions. Torsion bars were used in the 1960s for rear suspensions. Independent rear suspensions often feature steel coil springs. Springs generally experience only a primary single stress during flexure in operation. Torsion bars and coil springs primarily experience torsion, while leaf springs primarily experience bending stress.
Steel leaf springs have a very high modulus of elasticity. Thus, a single thick steel leaf spring is too stiff for automotive vehicle use. In a given application where a single glass fiber/epoxy resin spring can be used, a multi-leaf steel spring comprising ten thin steel leaves must be used to provide the requisite spring rate.
Leaf springs have been made of carbon fibers molded together in an epoxy matrix. Like steel, carbon fiber leaf springs are also too stiff to use as a single thick leaf spring, due to the high modulus of carbon fibers. Whereas a certain automotive vehicle application can use a single glass fiber leaf spring, three or four thin carbon fiber leaves must be used. Thus, although carbon fiber composites are ideal for use in vehicle structural parts since they are stiff and can be formed into structural shapes (e.g., H, U, I, L, T, etc.), this very stiffness makes them unsuitable for single leaf springs.
Composite leaf springs constructed of glass fiber in an epoxy matrix have been used to replace steel leaf springs in high volume production of passenger cars and vans in recent years. For conventional cars and vans, composite leaf springs offer the advantage of improved packaging in addition to reduced weight. For example, in one prototype front-wheel drive station wagon, substituting glass fiber/epoxy rear leaf springs for the normal steel coil spring suspension resulted in increased rear passenger space. For sedan models, this same change would translate into increased luggage space. In the front suspension, it has been demonstrated that installation of a transverse composite leaf spring occupies less space in critical areas, enabling use of a lower hood line, which improves the aerodynamic efficiency of the car.
Additional benefits achieved by using these composite leaf springs are the high durability and the high elastic strain energy inherent in a glass fiber/epoxy resin construction. The elastic strain energy of glass fiber/epoxy resin is more than ten times that of steel. Composite leaf springs offer a 25% to 75% weight reduction over conventional steel springs and have a strength-to-weight ratio up to five times greater than that of steel springs.
In the area of suspension system design, composite leaf springs enable a reduction in component size or, in some cases, enable elimination of stabilizer bars, strut assemblies, control arms and other components. Vehicle ride and handling characteristics are also improved because the composite leaf springs offer increased roll rates while also allowing lower ride rates.
One of the first vehicles utilizing composite leaf springs as its rear suspension system was the 1981 Corvette. This composite leaf spring was used on all Corvettes having automatic transmission and standard suspension, comprising about 80% of production or 40,000 cars per year. This spring is a single leaf spring 49 inches long, weighing 8 pounds, which replaced a ten leaf steel spring weighing 41 pounds. This enabled a weight reduction of 33 pounds or 80%.
The Corvette composite leaf spring is of constant cross-sectional area design that tapers from a 21/4 inches by 1 inch center section to a nearly 5 inches by less than 1/2 inch section at each end. The spring is mounted laterally at the rear of the car and performs like two-back cantilever springs in supporting almost 2200 pounds of vehicle weight. Corvette uses this unusual design because there is insufficient space to accommodate a normal coil spring independent rear suspension. These Corvette springs are produced by using an automated filament winding/compression molding process based on computerized filament and winding equipment. The basic manufacturing process for composite leaf springs is known and described, for example, in U.S. Pat. No. 2,600,843 and U.S. Pat. No. 3,142,598.
Presently, most composite leaf springs are prepared at the same spring rate. However, ride improvements have been experienced in vehicles having composite leaf springs of reduced spring rates. By using beam theory and small deflection, the spring rate of a leaf spring defines as a ratio between spring load and deflection; under four-point or three-point bending it is described by the following governing equation: EQU R=P/D=4.times.E.times.(bh.sup.3 /L.sup.3).times.(3K-4K.sup.3).sup.-1
where R is spring rate, P is pressure, D is distance, E is material modulus, b is beam width, h is beam thickness, L is beam length, and K is loading location. As shown in the equation, the spring rate is a function of material modulus, spring geometry and the distance between loading and supporting points.
From the equation, it is seen that spring rate is sensitive to the spring geometry, especially thickness and length. Spring rate is proportional to the cubic power of the thickness-to-length ratio. However, varying spring rate by changing spring geometry (shape) is economically impractical for large-scale production because providing different spring shapes requires constantly retooling to provide different mold designs.
Spring rate is directly proportional to the stiffness or modulus of the material. For unidirectional composites, the modulus in the fiber direction can be calculated by the following equation: EQU E=.SIGMA.E.sub.i V.sub.i
where E and V are the modulus and volume fraction, respectively, of constituent i in the composite.
The load-carrying capability of a spring is the product of spring rate and deflection. Instead of varying the camber height or deflection, spring rate can be varied to produce a new load-carrying capability. Test results for two differently-shaped composite leaf springs, denoted A and B, which are designed for different load carrying requirements in a single vehicle, are shown in FIG. 2. The phantom line in FIG. 2 represents a hypothetical 17 N/mm rate spring for an A spring design. This spring has the same design load as springs of the B spring design.
The plot of load-carrying capabilities of the springs against deflection in FIG. 2 illustrates that deflection increases with load. The slope of the load-deflection line is the spring rate, which determines the stiffness of the spring. Since both springs have the same rate, 26 N/mm, their load deflection lines are parallel. Load-carrying capability is determined by camber height (i.e., the deflection from free shape to flat-out position.) Since spring B has a lower load-carrying requirement, a smaller camber height is used. Spring A utilizes a higher camber height to accommodate higher load requirements.
Where a softer vehicle ride is desired, a composite leaf spring that has a lower spring rate, i.e., lower flexural stiffness, can be used. There are several known ways to reduce the flexural stiffness of a composite leaf spring. One is to reduce the glass fiber content of the composite spring.
Sample composite leaf springs A and B contain 55 volume percent of glass fiber and 45 volume percent of epoxy. Since glass fiber has a much higher modulus than the epoxy resin matrix material, the composite modulus is directly proportional to the volume fraction of the glass fiber. The composite modulus of the leaf spring can be varied by varying the content of glass fiber. However, in practice, it is very difficult to produce a homogeneous composite spring having a low glass fiber content by using the filament winding process. Such a spring inevitably has resin-rich areas having lower strength and fiber-rich areas which are stress-concentrated, both of which lower performance of such a composite spring.
The spring rate of a composite leaf spring can be changed by changing the dimensions of the spring. This is an expensive solution since major rework of the spring winding tooling would be required. Also, this may be impractical, since great dimensional changes may be needed but may not be possible in a given vehicle application.
Also, spring rate can be reduced by replacing some of the glass fiber content with a fiber having a lower modulus than glass fiber. However, it is difficult to change the glass fiber content greatly in a filament winding process because of process constraints. Layering of different modulus fibers in a leaf spring has been tried, but this produces a spring subject to catastrophic operational failure because of delamination caused by non-uniform stress.
It would, therefore, be desirable to provide a composite leaf spring with a reduced spring rate and uniform stress, which utilizes glass fibers and other fibers and does not delaminate.
It would also be desirable to provide a method of making a hybrid composite leaf spring with a reduced spring rate which evenly distributes glass fibers and other fibers throughout a resin matrix.
It would be further desirable to provide a method of making a fiber-reinforced composite leaf spring having a reduced spring rate without modifying the molding tooling.
It would be yet further desirable to provide a method of making composite leaf springs which can have varying spring rates for a given spring dimension by changing the mix of different modulus fibers.